Importance : The optimal staging for gastric cancer remains a matter of debate. Objective : To evaluate the value of 18 F-fludeoxyglucose-positron emission tomography with computed tomography (FDG-PET/CT) and staging laparoscopy (SL) in addition to initial staging by means of gastroscopy and CT in patients with locally advanced gastric cancer. Design, Setting, and Participants : This multicenter prospective, observational cohort study included 394 patients with locally advanced, clinically curable gastric adenocarcinoma (=cT3 and/or N+, M0 category based on CT) between August 1, 2017, and February 1, 2020. Exposures : All patients underwent an FDG-PET/CT and/or SL in addition to initial staging. Main Outcomes and Measures: The primary outcome was the number of patients in whom the intent of treatment changed based on the results of these 2 investigations. Secondary outcomes included diagnostic performance, number of incidental findings on FDG-PET/CT, morbidity and mortality after SL, and diagnostic delay. Results : Of the 394 patients included, 256 (65%) were men and mean (SD) age was 67.6 (10.7) years. A total of 382 patients underwent FDG-PET/CT and 357 underwent SL. Treatment intent changed from curative to palliative in 65 patients (16%) based on the additional FDG-PET/CT and SL findings. FDG-PET/CT detected distant metastases in 12 patients (3%), and SL detected peritoneal or locally nonresectable disease in 73 patients (19%), with an overlap of 7 patients (2%). FDG-PET/CT had a sensitivity of 33% (95% CI, 17%-53%) and specificity of 97% (95% CI, 94%-99%) in detecting distant metastases. Secondary findings on FDG/PET were found in 83 of 382 patients (22%), which led to additional examinations in 65 of 394 patients (16%). Staging laparoscopy resulted in a complication requiring reintervention in 3 patients (0.8%) without postoperative mortality. The mean (SD) diagnostic delay was 19 (14) days. Conclusions and Relevance : This study's findings suggest an apparently limited additional value of FDG-PET/CT; ...